1. Field of the Invention
This invention relates to fiber optic connectors and specifically to testing the intermateability of fiber optic connectors.
2. Background Information
Fiber optic connectors are devices that transfer optical power between optical fibers but which need to be connected and disconnected repeatedly. Fiber optic connectors are connected by mating a connector plug with an adapter to form a connector assembly. In other words, a connector assembly is formed when two connector plugs are inserted into the opposite ends of an adapter. The connector plug is the male member of the connector assembly and is mounted onto the end of the optical fiber. The adapter is the female member of the connector assembly. Since the advent of fiber optics several different types of connectors have been designed and made commercially available. These different connector types are identified by their commercial designation and include those commercially designated as Biconic, FC, and SC. If the two plugs of a connector assembly are of the same type then both ends of the adapter are of same type or commercial designation. On the other hand, hybrid adapters having different openings for different connector types are available and are used to intermate different connector plug types.
Critical to the design of any fiber optic connector is the ability of the connector to provide minimal loss and reflection when mated or connected to another connector of the same type. In order to minimize the loss and reflection of a connection, the Telecommunications Industries Association/Electronic Industries Association (TIA/EIA), along with other national standard bodies, has promulgated design specifications for each type of plug and adapter. These specifications place limits on various physical plug and adapter parameters so as to achieve a prespecified or common level of performance. These physical parameter limits include, but are not limited to, the connector plug ferrule diameter tolerance, the inside diameter of the connector adapter sleeve and the contact angle of the connector plug.
As with any design specification, parameters are specified with tolerance margins that, if met, should provide the prespecified level of performance. Of course, when any item is mass produced, such as connectors, some are bound to fall outside the tolerance margins. Furthermore, in the case of fiber optic connectors, not only is intermateability performance required among connectors from the same supplier or manufacturer, but connectors are often mated among different suppliers. In fact, fiber optic connectors are regarded as off shelf items throughout the fiber optic industry and are expected to intermate and meet the prespecified levels of performance without regard to manufacturer. As such, there is a need to test connectors against industry standards to gather information on performance and reliability. In fact, fiber optic connector suppliers, as part of their quality control program, normally test fiber optic connectors so as to compile reliability data on their manufacturing operation. Purchasers of fiber optic connectors also maintain quality programs, such as testing samples of the connectors against various industry standards, so as to obtain data on the performance of the connectors that are employed in the purchaser's products or network facilities.
TIA/EIA has documented a series of intermateability standards as part of its TIA/EIA 604 "Fiber Optic Intermateability Standards (FOCIS)". Of particular import to this invention is TIA/EIA's documents 604-2, so called FOCIS-2, 604-3, FOCIS-3, and 604-4, FOCIS-4. FOCIS-2 is directed to connectors with the bayonet Fiber Optic Connectors (BFOC/2.5) commercial designation. FOCIS-3 and FOCIS-4 are directed to connectors with the SC and FC commercial designations, respectively. The, FOCIS-2, FOCIS-3, and FOCIS-4 documents specify requirements that, if met, should ensure that any combination (including hybrids) of each type of connector will mechanically intermate and will meet a common level of performance. In a nutshell, the FOCIS-2/3/4 documents require plugs and adapters with the same commercial designation to intermate while meeting prespecified levels of performance. Specifically, section 2.2 of the FOCIS-2/3/4 documents defines the adapter standard and section 3 defines the plug standards.
Among the plug intermateability requirements is a requirement on the distance the connector plug ferrule is allowed to travel relative to the body of the plug in response to a contact force exerted against the ferrule. These requirements are referred to in the art as the ferrule extension and contact force requirements. The connector plug ferrule is a mechanical fixture, generally a rigid tube, used to confine the stripped end of an optical fiber or cable. The movement of the ferrule is normally restricted by use of a spring and therefore the requirements are directed to both the amount of force required to displace the ferrule spring a certain distance and the normal rest length of the ferrule. A ferrule spring loading force, F, is specified for different distances, A, where A is defined as the distance of the mechanical reference plane of a connector plug-adapter assembly to the tip of the connector plug ferrule. FIG. 1A illustratively depicts the relationship of F and A for a connector plug 11 having a ferrule 12. For example, if no force, i.e., F=0, is exerted on the ferrule then the spring is expected to extend the ferrule its farthest distance, i.e., the normal rest length or no contact force length of the ferrule. As such, the rest length of an SC type connector ferrule is required to be greater than or equal to 7.15 mm, whereas for an FC type connector ferrule the rest length is required be greater than or equal to 3.8 mm. The full relationship, as given by the TIA/EIA, between A and F is given in Tables I and II below for SC and FC connector types, respectively, where A is measured from a reference plane 13 to the tip of ferrule 12. The requirements on other connectors are contained in other FOCIS documents.
TABLE I ______________________________________ EIA/TIA SC Type Ferrule Travel and Contact Force Requirements IF THEN ______________________________________ F = 0 A .gtoreq. 7.15 mm A .ltoreq. 7.1 mm F .gtoreq. 7.8 Newtons (N) A .gtoreq. 6.9 mm F .ltoreq. 11.8 N ______________________________________
TABLE II ______________________________________ EIA/TIA FC Type Ferrule Travel and Contact Force Requirements IF THEN ______________________________________ F = 0 A .gtoreq. 3.8 mm A .ltoreq. 3.7 mm F .gtoreq. 7.8 N A .gtoreq. 3.6 mm F .ltoreq. 11.8 N ______________________________________
Apparatus to measure the spring loaded force applied to a ferrule when fiber optic connector plugs are mated with each other has been proposed by the International Electrotechnical Commission (Document Title "IEC 1330-3-22: Fibre Optic interconnecting devices and passive components--Basic test and measurement procedures", final draft submitted Nov. 1, 1996). However, in order to assure the intermateability of fiber optic connectors, particularly from different manufacturers, I have found that another critical adapter dimension requirement is the adapter length. The adapter length, E, is defined as the distance from one mechanical plane 17 to the midpoint 18 between the two mechanical reference planes 17, 19 of a simplex adapter 310 as illustratively shown in FIG. 1B. For an SC type connector the adapter length, E, is specified as 6.9 millimeter (mm).ltoreq.E.ltoreq.7.1 mm. For a FC type connector E is specified as 3.6 mm.ltoreq.E.ltoreq.3.7 mm.
The adapter length and the ferrule extension and contact force requirements are directed to the intermateability between adapters and plugs of the same commercial designation. For example, if the adapter length is longer than required, then the ferrules of two intermated plugs will not be close enough to each other in the connector assembly to meet performance requirements. Thus, apparatuses and methods are needed that can be used to test the adapter length in addition to the ferrule extension and contact force of a connector against the requirements given in TIA/EIA documents FOCIS-3 and FOCIS-4 so as to ensure intermateability.